Method and apparatus for eliminating noises with a read operation of magnetic disk unit

ABSTRACT

A processor of a drive unit positions a head to a designated cylinder position of a disk medium on the basis of a command notified from an upper controller through a data bus and amplifies a read signal by a read amplifier and, after that, transmits the amplified read signal to the upper controller through a read signal line. A data disconnecting circuit is provided for the data bus from an upper apparatus to the processor. The data disconnecting circuit switches the data bus to a disconnection state by a control signal which is generated from the upper apparatus during the data reading operation. When the control signal is interrupted by the end of the data reading operation, the data bus is returned to the connection state.

BACKGROUND OF THE INVENTION

The present invention relates to noise eliminating apparatus and methodof a magnetic disk unit for preventing that noises are mixed into a readsignal which is read out from a disk medium and is transmitted to anupper apparatus and, more particularly, to noise eliminating apparatusand method of a magnetic disk unit for preventing that noises which aregenerated by the operation of a data bus for connecting a drive unitwith an upper apparatus are mixed into a read signal.

In a magnetic disk subsystem which is used as an external memoryapparatus of a computer, one or a plurality of drive units are connectedunder the domination of an upper disk controller. As such a subsystem,there are a concentrated processing type and a distributed processingtype. In the concentrated processing type, a formatter is provided forthe upper controller and, in the reading mode, an analog read signal isgenerated from the drive unit, thereby reconstructing read data on thecontroller side. In the distributed processing type, a formatter isprovided for each drive unit and all of the read data is reconstructedby the drive unit and is transmitted to the upper apparatus. The presentinvention belongs to the former concentrated processing type.

In the case where the formatter section is provided for the uppercontroller, when receiving a seek command from the upper controller, thedrive unit moves a head to a designated cylinder address by the drivingof an actuator by a voice coil motor. The actuator for positioning thehead is generally controlled by a micro processor or a digital signalprocessor.

When the drive unit notifies the completion of the seeking operation tothe upper controller, an analog write signal is transmitted by a writesignal line in the writing mode and is supplied to a write head and iswritten into a disk medium. In the reading mode, when receiving aconfirmation notification from the upper controller for the seekcompletion response, a read signal of a read head (MR head) is amplifiedby a reading amplifier and is transmitted to the upper controller by aread signal line.

FIG. 1 is a schematic diagram of a conventional magnetic disk drive unitcomprising: a disk enclosure 100; a reading amplifier board 102; and adrive analog board 104 on which a processor 106 is installed. The diskenclosure 100 is a mechanism portion having a disk medium, a head, anactuator, a voice coil motor, a spindle motor, and a head IC. A readingamplifier with an AGC function is installed on the reading amplifierboard 102. In addition to the processor 106, a D/A converter, an A/Dconverter, a servo demodulating circuit, and the like are installed onthe drive analog board 104. The processor 106 on the drive analog board104 is connected with the upper controller by a read signal line 108, awrite signal line 109, and a data bus 110 and transmits and receivesvarious kinds of commands, the write signal, and the read signal.

The head provided for the disk enclosure 100 integrally supports a writehead and a read head at the edge of the actuator. Hitherto, a magnetichead has been used as each head. On the other hand, in recent years, asmall MR head using a magnetoresistive device is used as a read head inorder to improve a track recording density. The MR head can beminiaturized as compared with the magnetic head and also has a highmagnetic converting efficiency. Assuming that those heads have the sameintensity of magnetic field, the MR head generates a read voltage higherthan that of the magnetic head. In order to make the MR head operativeas a magnetoresistive device, it is necessary to supply a predeterminedbias current in the reading mode. For this purpose, a bias voltage ofabout 2.0 V is supplied from the drive analog board 104 to the diskenclosure 100 via a bias signal line 112, thereby applying the biasvoltage to the MR head. Further, when the MR head to which the biasvoltage was applied is come into contact with the disk medium, anexcessive short-circuit current flows, thereby causing a head breakage.Therefore, by also supplying the same bias voltage to both of the diskmedium and the casing side, they are set to the same potential and ahead breakage is prevented.

The realization of a miniaturization and a high density of the magneticdisk unit is remarkable in recent years. The inventors of the presentinvention has examined to miniaturize the drive unit of FIG. 1, so thata drive unit in which an area of a printed circuit board is reduced toabout the half could be developed as shown in FIG. 2. In this case, inaddition to the miniaturization of a disk enclosure 114, a readingamplifier 118 is installed on a drive analog board 116 together with aprocessor 120.

In the miniaturized drive unit in FIG. 2, however, there occurs aproblem such that fairly large noises 122 are periodically mixed to aread waveform 124 in FIG. 3B which is generated in an active state of aread gate in FIG. 3A in the upper controller, so that read data cannotbe accurately reconstructed. By examining a generation source of thenoises 122 which are mixed to the read waveform 124, it has been foundthat the noises were generated synchronously with the switchingoperation of a data bus in FIG. 3C. Such noises of the read waveformdon∝t cause a problem in the drive unit in FIG. 1 and appear for thefirst time by remarkably miniaturizing as shown in FIG. 2.

The first cause of the mixture of the noises is that the bias voltage isapplied to the MR head. The bias voltage of about 2.0 V is applied fromthe drive analog board 116 to the MR head of the disk enclosure 114. Inassociation with the miniaturization of the circuit board, the noisesgenerated by the operation of the data bus are mixed to the bias voltagegenerated in the bias voltage generating circuit and are supplied to theMR head, so that the noises appear in the read signal. Although anoutput voltage of the MR head is generally equal to about 0.5 mV, noisesof about 50 mV which is 100 times as large as such an output voltage aremixed according to the actual measurement. A head IC provided for thedisk enclosure 114 uses a differential circuit to prevent noises.However, in case of such large noises of 50 mV, a difference betweennoises mixed to two differential signal lines appears in an output.

The second cause of the mixture of noises is that since the area of theprinted circuit board is reduced, a distance between the read signalline 108 and the data bus 110 decreases and noises by crosstalk areeasily mixed. Since the noises by the crosstalk increase in proportionto the square of the distance between the signal lines, it is consideredthat such a short distance becomes a cause.

SUMMARY OF THE INVENTION

According to the invention, there is provided a magnetic disk apparatushaving a high reliability in which noises by the operation of a data busare not mixed to a read signal and no read error occurs even if acircuit board is miniaturized.

According to the invention, a drive unit has a processor for positioninga head to a designated cylinder position of a disk medium on the basisof a command notified from an upper apparatus, for reading outinformation, and for transmitting an analog read signal to the upperapparatus via a read signal line. The processor and the upper apparatusare connected by the data bus. In order to prevent that the noisesgenerated by the operation of the data bus are mixed into the readsignal, a data bus disconnecting circuit is provided for the data busfor connecting the processor and the upper apparatus. The data busdisconnecting circuit switches the data bus to a disconnection state bya control signal which is outputted from the upper apparatus during thedata reading operation. When the control signal is interrupted by thecompletion of the data reading operation, the disconnecting circuitswitches the data bus to a connection state. That is, the data bus isswitched to the disconnection state by a control signal which isgenerated in an active state of a read gate in the upper apparatus.

A microprocessor of the drive unit moves an actuator on the basis of aseek command, thereby moving the head to a necessary read cylinderposition. For this purpose, during the reading operation, the head ison-track controlled to the read cylinder and it is unnecessary toreceive the seek command from an upper controller. Therefore, even ifthe data bus is disconnected during the reading operation in which thereis no need to move the actuator, no problem will occur. When themagnetic disk finishes the reading operation, the data bus is returnedto the original connection state and the microprocessor receives thecommand and is set to a state in which the actuator can be controlled.

In the case where an error occurs during the data reading operation, theprocessor of the drive unit generates an error generation interruptionsignal to the upper apparatus and the data bus disconnecting circuit. Inthis instance, when receiving the error generation interruption signalfrom the processor in the disconnection state of the data bus, the datadisconnecting circuit returns the data bus to the connection state.

Further, the processor of the drive unit generates a bus release requestsignal to the data bus disconnecting circuit when receiving a commandnotification signal indicating that a command is generated from theupper apparatus during the data reading operation. When receiving thebus release request signal from the processor in the disconnection stateof the data bus, the data bus disconnecting circuit returns the data busto the connection state.

The drive unit of the present invention comprises: a drive controllerhaving a processor; and a disk enclosure having a disk medium, a head, ahead actuator, and a spindle motor. The head integrally includes a writehead and a read head. At least the read head is an MR (magnetoresistive)head. A predetermined bias voltage is applied to both of the MR head andthe disk medium from the drive controller side. As a processor of thedrive unit, for example, a digital signal processor is used.

According to a noise eliminating method of the magnetic disk apparatusof the invention, the data bus for connecting the processor and theupper apparatus is switched to the disconnection state on the basis of acommand notified from the upper apparatus for a period of time duringwhich a read signal is being transmitted from the drive unit having theprocessor via the read signal line. After completion of the data readingoperation, the data bus is returned to the connection state. Thus, it isprevented that the noises generated by the operation of the data busduring the data reading operation are mixed into the read signal fromthe head.

The switching operation to the disconnection state of the data bus inthe drive unit is performed by the control signal generated in theactive state of the read gate in the upper apparatus, In the case wherea read error occurs in the drive unit, the data bus in the disconnectionstate is returned to the connection state. In the case where the driveunit receives the command notification signal indicating that a commandis generated from the upper apparatus during the data reading operation,the data bus in the disconnection state is also returned to theconnection state.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of a board construction of aconventional drive unit;

FIG. 2 is an explanatory diagram of a board construction of aminiaturized drive unit;

FIGS. 3A to 3C are timing charts of the mixture of noises to a readwaveform in association with the miniaturization in FIG. 2;

FIG. 4 is a block diagram of a hardware construction of the invention;

FIG. 5 is an explanatory diagram of a structure of a disk enclosure inFIG. 4;

FIG. 6 is a side sectional view of a head actuator in FIG. 5;

FIG. 7 is a circuit block diagram of a data bus disconnecting circuit inFIG. 4; and

FIGS. 8A to 8D are timing charts of the noise elimination by adisconnection of a data bus of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 4, a magnetic disk drive unit of the invention comprises a diskenclosure 10 and a drive controller 12. A spindle motor 14 for rotatinga disk and a voice coil motor (hereinbelow, referred to as a "VCM") 16for activating a head are provided for the disk enclosure 10. In orderto read out information of the servo surface of the magnetic disk, aservo head 18 and a servo head IC 22 are provided. Further, in order toread and write information of a plurality of data surfaces, data heads20-1 to 20-19 and a data head IC 24 are provided. Each of the data heads20-1 to 20-19 integrally includes a write head and a read head in a headportion. A magnetic head is used as a write head and an MR head using amagnetoresistive device is used as a read head. The MR head performs thereading operation in a state in which a bias current flows by supplyinga bias voltage from the drive controller 12 side in the readingoperation.

With respect to core widths of the servo head 18 and the write heads andread heads which are provided for the data heads 20-1 to 20-19, thereare relations such that the core width of the servo head 18 is largestand the core width of each of the write heads is subsequently large andthe core width of each of the read heads (MR heads) is smallest.Assuming that the track pitch of the data surface is set to, forexample, 7 μm, the core width of the servo head 18 is set to 7 μm whichis almost equal to the length of the track pitch. On the other hand, thecore width of the write head provided for the data head is equal to 6μm. The core width of the MR head as a read head is set to about 3 μmwhich is the half of the core width of the write head.

FIG. 5 shows an internal structure of the disk enclosure 10 in FIG. 4.Eleven magnetic disks 60-1 to 60-1 are rotatably built in a casing 62 ofthe disk enclosure 10 so as to be rotatable by the supporting of arotary axis 64 and are rotated by a spindle motor (not shown) providedin the lower portion. A head actuator 68 is arranged on the right sideof the magnetic disks 60-1 to 60-11 and enables a head attached at theedge to be integrally moved in the radial direction of the mediumsurface of each of the magnetic disks 60-1 to 60-11. In the embodiment,disks each having a diameter of 5.25 inches are used as magnetic disks60-1 to 60-11.

FIG. 6 is a vertical sectional view of the head actuator 68 in FIG. 5.In the head actuator 68, a block 72 is rotatably attached through upperand lower bearings 66-1 and 66-2 to a shaft 70 which is fixedlyprovided. A coil 74 of the VCM 16 is attached on the right side of theblock 72. Eleven arms 76-1 to 76-11 are integrally extended on the leftside of the block 72. Two heads are supported at the edge of each of thearms 76-1 to 76-11 through a pair of spring arms. In the embodiment,twenty heads are provided for the eleven magnetic disks 60-1 to 60-11.The upper nine heads among those heads are the data heads 20-1 to 20-9among those heads and, subsequently, the servo head 18 is provided. Theremaining ten heads subsequent to the servo head 18 are the data heads20-10 to 20-19. The disk surfaces of the magnetic disks 60-1 to 60-11which the data heads 20-1 to 20-19 face are set to data surfaces whichare used in the reading and writing operations of data. On the otherhand, a medium surface on the upper side of the magnetic disk 60-6 overwhich the servo head 18 is positioned is set to a servo surface on whichservo information has been recorded on all tracks. The reason why themedium surface of the central magnetic disk 60-6 is set to the servosurface is to minimize the distances from the central servo surface tothe farthest magnetic disks 60-1 and 60-1, thereby minimizing an offsetof the data surface by a mechanical deformation due to a temperaturechange.

Although a predetermined bias voltage is applied to each of the MR headsprovided for the data heads 20-1 to 20-19, for example, the bias voltagecan be applied by connecting a supply line of the bias voltage to theblock 72 made of aluminum. Further, since the bias voltage is applied tothe MR heads, when the heads come into contact with the magnetic disks60-1 to 60-11, the heads are broken by a short-circuit current. Toprevent such a situation, the same bias voltage is also applied to themagnetic disks 60-1 to 60-11. The bias voltage can be applied to themagnetic disks 60-1 to 60-11 by, for example, connecting the biasvoltage supply line to a fixed portion of the rotary axis 64 in FIG. 5.

Referring to FIG. 4 again, the drive controller 12 will now bedescribed. A digital signal processor (referred to as a "DSP") 26 tofunction as a drive processor is provided for the drive controller 12.The DSP 26 receives a seek command from a microprocessor unit (referredto as an "MPU") 28 of an upper control unit 15 and controls the voicecoil motor 16 of the disk enclosure 10, thereby positioning each head tothe cylinder address designated in the magnetic disk by the headactuator 68 in FIG. 5. A position signal forming circuit 36 for forminga head position signal is provided for the head positioning control ofthe DSP 26. A read signal of the servo head 18 is supplied to theposition signal forming circuit 36. In the embodiment, servo informationis recorded on the data surface of the disk medium and the positionsignal forming circuit 36 forms a position signal indicative of the headposition on the basis of the read signal of the phase servo information.The position signal from the position signal forming circuit 36 isconverted to the digital data by an A/D converter 38 and is sent to theDSP 26. The DSP 26 controls the spindle motor 14 through a D/A converter32 and a driver 34 and drives the VCM 16 through a D/A converter 40 anda driver 42, thereby performing the positioning control of the head bythe rotation of the head actuator 68 in FIG. 5.

The head positioning control by the DSP 26 is divided to a seek controlfor moving the head to a target cylinder on the basis of the seekcommand and an on-track control for maintaining the on-track state bythe arrival of the head to the target cylinder. The seek controlcomprises a coarse control and a fine control. The coarse control is acontrol for moving the head to a position just before the targetcylinder in accordance with a target speed pattern. The fine control isa control for switching the control mode from a speed control to aposition servo control and for positioning the head to the targetcylinder when the head reaches a position just before the targetcylinder, for example, 0.5 cylinder before by the coarse control.Therefore, after the seek command was received from the upper controlunit 15, the DSP 26 doesn∝t need to receive the seek command inprinciple until the reading operation or writing operation is finished.

On the other hand, in order to read or write from/to the data surface ofthe magnetic disk, a reading amplifier 44 and a writing amplifier 48 areprovided. During the reading operation, the data head IC 24 selects thehead of the head address designated from the upper control unit 15 by aswitching signal from the DSP 26. The read signal read out from theselected head is amplified by a built-in pre-amplifier and, after that,it is supplied to the reading amplifier 44. The reading amplifier 44amplifies the read signal to the signal level which is necessary totransfer to the upper control unit 15 and, after that, transmits theamplified signal to a formatter section 52 of the upper control unit 15through a read signal line 46. A VFO, an equalizer section, and a datademodulating section are provided for the formatter section 52 incorrespondence to the read signal obtained from the read signal line 46,thereby reconstructing the read data from the received read signal andoutputting.

A write signal formed on the basis of write data is supplied from theformatter section 52 of the upper control unit 15 to the writingamplifier 48 through a write signal line 50. The input write signal isamplified by the writing amplifier 48 and, after that, it is supplied tothe head selected by the instruction from the DSP 26 at that timethrough the data head IC 24, and is written to the magnetic disk.

Further, a bias current control circuit 54 is provided for the driveunit 12. The bias current control circuit 54 receives a control signalfrom the DSP 26 in the reading operation and supplies a bias voltage tothe read head (MR head) through the data head IC, thereby allowing abias current to flow.

According to the invention, in such a drive unit, a data busdisconnecting circuit 30 is provided for the data bus for connecting theDSP 26 of the drive controller 12 and the MPU 28 of the upper controlunit 15. That is, the DSP 26 is connected to the data bus disconnectingcircuit 30 through a data bus 58 and the data bus disconnecting circuit30 is connected to the MPU 28 of the upper control unit 15 through adata bus 56. Further, a command notification signal E1 for notifying thegeneration of a command is supplied from the MPU 28 to the DSP 26 by acontrol line 55-1. In order to control the data bus disconnectingcircuit 30, a data bus disconnection signal E2 is supplied from the MPU28 by a control line 55-2. The data bus disconnection signal E2 is heldat the H level for a period of time during which the read gate is set tothe active state by the reading operation by the MPU 28. When the databus disconnection signal E2 from the MPU 28 is set to the H level, thedata bus disconnecting circuit 30 disconnects the connection statebetween the data buses 56 and 58. When the data bus disconnection signalE2 is returned to the L level, the data buses 56 and 58 are connectedagain.

A control line 55-3 is connected from the DSP 26 to the data busdisconnecting circuit 30. In the case where an error occurs on the drivecontroller 12 side during the reading operation, the control line 55-3generates an error generation interruption signal E3. The control line55-3 is further branched as a control line 55-4 and is connected to theMPU 28 of the upper control unit 15, thereby supplying the same errorgeneration interruption signal E3.

Further, a control line 55-5 is connected from the DSP 26 to the datadisconnecting circuit 30, thereby supplying a bus release request signalE4. The bus release request signal E4 is provided for the case offorcedly performing the seeking operation by a command from the uppercontrol unit 15 even during the reading operation in a state in whichthe data buses are disconnected. That is, when the DSP 26 receives thecommand notification signal E1 indicating that the seek command isoutputted through the control line 55-1 during the reading operation,the DSP 26 supplies the bus release request signal E4 to the data busdisconnecting circuit 30, thereby connecting the data buses 56 and 58and returning to a state in which a command can be received by the MPU28. As a forced seeking operation which needs the output of the busrelease request signal E4 during the reading operation, for example,there is a device resetting operation. The device resetting operation isa process for resetting the drive unit and returning to the initialstate, for example, in the case where a program of the MPU 28 of theupper control unit 15 runs away. Since the head is positioned to the 0cylinder in order to return the drive unit side to the initial state,when recognizing the device resetting state during the readingoperation, the DSP 26 generates the bus release request signal E4,thereby returning the data bus disconnecting circuit 30 to theconnection state of the data buses 56 and 58 and enabling the seekcommand from the MPU 28 to the 0 cylinder to be received.

FIG. 7 shows a practical embodiment of the data bus disconnectingcircuit 30 in FIG. 4. The data bus disconnecting circuit comprises: aD-FF 82; a control circuit 86; and a driver/receiver having switchingcircuits 88-1 and 88-2. As a driver/receiver 90, for example, an LS245made by Fujitsu Ltd. can be used. A clock signal E5 for a latchingoperation is supplied to a clock terminal CLK of the D-FF 82. The databus disconnection signal E2 is supplied to a data terminal D. The errorgeneration interruption signal E3 or bus release request signal E4 issupplied to a clear terminal CLR through an OR gate 84.

When the data bus disconnection signal E2 is set to the H level in theactive state, the signal E2 is latched to the D-FF 82 at a leadingtiming of the clock signal E5, so that a Q output is set to the H level.The Q output of the D-FF 82 is supplied to a control terminal G of thecontrol circuit 86. When the Q output is at the H level, the controlcircuit 86 sets the switching circuits 88-1 and 88-2 to thedisconnection state, thereby disconnecting the data buses 56 and 58.When the Q output is set to the L level, the control circuit 86 sets theswitching circuits 88-1 and 88-2 to the connection state, therebyconnecting the data buses 56 and 58. The command notification signal E1is supplied to enable terminals EN1 and EN2 of the control circuit 86.When the command notification signal E1 is at the L level, it forms asignal transmission state from the MPU 28 of the upper control unit 15to the DSP 26 of the drive controller 12. When the command notificationsignal E1 is at the H level, it forms a signal transmission state fromthe DSP 26 of the drive controller 12 to the MPU 28 of the upper controlunit 15.

Specifically speaking, each of the switching circuits 88-1 and 88-2 hasa bidirectional transmitting circuit in which a driver and a receiverare connected in parallel every data bus line. When the commandnotification signal E1 is set to the L level, the receiver side is setto an enable state, thereby enabling the signal transmission from theMPU 28 to the DSP 26. When the command notification signal E1 is set tothe H level, the driver side is set to the enabling state, therebyenabling the signal transmission from the DSP 26 to the MPU 28. Further,in the data bus disconnection state in which the Q output of the D-FF 82is set to the H level, the enable states on both of the driver side andthe receiver side are released, thereby forming the data busdisconnection state.

FIGS. 8A to 8D show a situation of the noise elimination by the datadisconnecting circuit 30 provided for the drive controller 12. In thereading operation, as shown in FIG. 8A, a read gate is set to an activestate at the H level during the reading operation by the MPU 28 of theupper control unit. In association with the active state of the readgate, the data bus disconnection signal E2 is set to the H level and isoutputted by the MPU 28 of the upper control unit 15 through the controlline 55-2. Thus, the data buses 56 and 58 connecting the MPU 28 and theDSP 26 are set to the disconnection state during the reading operationand the switching operations of the data buses are stopped as shown inFIG. 8D. Therefore, as shown in FIG. 8C, noises 92 which have alreadybeen mixed synchronously with the switching of the data buses before thereading operation are not mixed during the reading operation in the databus disconnection state. Only a read waveform 94 is obtained andoutputted to the formatter section 52 of the upper control unit 15through the read amplifier 44. As mentioned above, since the noises inassociation with the data bus operation are not mixed to the readwaveform 94, the read data can be accurately reconstructed in theformatter section 52 of the upper control unit 15.

After completion of the reading operation, the data bus disconnectionsignal E2 is set to the L level and the data bus disconnecting circuit30 release the disconnection state and returns to the original state.The data buses 56 and 58 are connected again, thereby starting the busoperation.

In the above embodiment, although the DSP 26 has been used as a drivecontroller 12, an MPU can be also used in a manner similar to the uppercontrol unit 15. The present invention is not limited by the numericalvalues shown in the embodiment.

What is claimed is:
 1. A noise eliminating apparatus of a magnetic disktrait, comprising:a drive unit which has a processor for positioning ahead to a cylinder address of a disk medium designated by a seek commandnotified from an upper apparatus and which transmits an analog readsignal read out from said disk medium in a head positioning state ofsaid processor through a read signal line; a data bus for connectingsaid processor and said upper apparatus; and a data bus disconnectingcircuit, inserted in said data bus, for switching said data bus for aperiod of time during which a read control signal is generated from saidupper apparatus while said disk medium is being read and for returningsaid data bus to a connection state when said read control signal isinterrupted; wherein said processor generates an error generationinterruption signal to said upper apparatus and said data busdisconnecting circuit when an error generation is detected during thereading operation, and said data bus disconnecting circuit returns saiddata bus to the connection state when said error generation interruptionsignal is received from said processor in a disconnection state of saiddata bus.
 2. An apparatus according to claim 1, wherein said data busdisconnecting circuit switches said data bus to the disconnection stateby a control signal which is generated in an active state of a read gatein said upper apparatus.
 3. An apparatus according to claim 1,whereinsaid processor generates a bus release request signal to saiddata bus disconnecting circuit in the case where a command notificationsignal indicating that a command is generated from said upper apparatusis received during the data reading operation, and said datadisconnecting circuit returns said data bus to the connection state inthe case where the bus release request signal is received from saidprocessor in the disconnection state of said data bus.
 4. An apparatusaccording to claim 1, wherein said drive unit comprises: a drivecontroller having said processor; and a disk enclosure including a diskmedium, a head, a head actuator, and a spindle motor,and wherein saidhead integrally has a write head and a read head, at least said readhead is an MR (magnetoresistive) head, and a predetermined bias voltageis applied to said MR head and said disk medium from said drivecontroller side.
 5. An apparatus according to claim 1, wherein saidprocessor of said drive unit is a digital signal processor.
 6. A noiseeliminating method of a magnetic disk unit, comprising the stepsof:positioning a head to a cylinder address of a disk medium designatedby a command notified from an upper apparatus; transmitting an analogread signal read out from said disk medium in said head positioningstate through a read signal line; and switching said data bus to adisconnection state for a period of time during which a read controlsignal is generated from said upper apparatus while said disk medium isbeing read and returning said data bus to a connection state when saidread control signal is interrupted; wherein an error generationinterruption signal is outputted to said upper apparatus and saiddisconnection state of said data bus is returned to the connection statewhen an error generation is detected during the reading operation.
 7. Amethod according to claim 6, wherein said data bus is switched to thedisconnection state by a control signal which is outputted in an activestate of a read gate in said upper apparatus.
 8. A method according toclaim 6, wherein when said drive unit receives a command notificationsignal indicating that a command is generated from said upper apparatusduring the data reading operation, said data bus in said disconnectionstate is returned to the connection state.
 9. A magnetic disk unitcomprising:an MR head for reading out recording information of a diskmedium; a bias current supplying circuit for supplying a bias current tosaid MR head when reading; a processor for positioning said MR head to acylinder address of a disk medium designated by a command which isnotified from an upper apparatus; a bus which is connected between saidupper apparatus and said processor and whose signal state is switchedfrom said upper apparatus to said processor or from said processor tosaid upper apparatus at predetermined clock intervals and whichtransfers a command; a read signal line, connected between said upperapparatus and said processor, for transmitting a read signal from saidprocessor to said upper apparatus; and a bus disconnecting circuit,imparted in said bus and having a bidirectional transmitting circuit inwhich a driver and a receiver are connected in parallel in oppositedirections, for releasing enable states of both of said driver and saidreceiver and forming a bus disconnection state when said read signal isbeing transmitted by said read signal line.